1. Field of the Invention
The present invention relates to a method and apparatus for modulating and driving a semiconductor laser, or the like, and a system, e.g., a recording apparatus, using the same.
2. Related Background Art
As a means for generating a light beam, a semiconductor laser is widely used in various systems since it is inexpensive and compact, and has a feature of directly performing strength modulation by a driving current.
As a drawback, however, the semiconductor laser has negative temperature characteristics exhibiting considerable driving current-light output characteristics. FIG. 42 shows the driving current-light output characteristics of the semiconductor laser (quoted from HL8312G Data Book, HITACHI). In FIG. 42, a driving current [mA] of the semiconductor laser is plotted along the abscissa, and a light output [mW] is plotted along the ordinate. Measurements were performed at case temperatures of 0.degree. C., 25.degree. C., and 50.degree. C. As can be read from the graph, negative temperature characteristics of about -0.1 mW/.degree. C. are observed. This implies that a light output largely varies depending on a variation in external temperature. Furthermore, a temperature of a semiconductor laser chip is increased due to an emission loss itself as an emission time elapses, and hence, a decrease in light output also occurs.
A laser beam printer is known as a commercially available system using a light source such as a semiconductor laser. For example, in the medical field, a laser printer for recording a medical image obtained by MR, CT, DSA, or the like onto a photosensitive recording medium such as a silver chloride film is widely used. A laser beam which is strength-modulated in proportion to a picture element density is deflected by a light deflector to attain main scanning, and a recording medium is moved in a direction perpendicular to the main scanning direction to attain sub-scanning, thereby recording a multi-gradation halftone image on the recording medium.
Since a laser printer normally performs recording at a main scanning speed of 1 to 2 msec and a rate of several seconds per page, an external temperature is left unchanged during at least one main scanning period, and a change in light output due to a change in temperature during this period is caused by a temperature rise due to an emission loss of the semiconductor laser itself.
As a means for compensating for a light output variation caused by the change in temperature, a circuit for continuously monitoring whether or not an emission level of the semiconductor laser coincides with a predetermined level (which is constant for a unit radiation time), and feeding the monitored level to a driving current, i.e., a so-called APC (Auto Power Control) circuit is generally used. This circuit is disclosed in detail in, e.g., U.S. Pat. Nos. 4,237,427, 4,412,331, 4,583,128, 4,625,315, and the like.
FIG. 43 is a block diagram of a basic APC circuit. In FIG. 43, a setup current 901 to be proportional to an emission amount is input to the APC circuit. The APC circuit includes a voltage adder 902, a voltage-to-current (V/I) converter 904 for converting a semiconductor laser driving voltage V.sub.d 903 to an actual driving current I.sub.d 905, a semiconductor laser 906, a PIN photodiode 907 for monitoring a laser emission amount, and a current-to-voltage (I/V) converter 909 for converting a monitor current I.sub.m 908 into a monitor voltage V.sub.m 910. In order to monitor a light output of the semiconductor laser 906 by the PIN photodiode 907, the PIN photodiode 907 monitors a back emission amount of the semiconductor laser at a trailing edge portion of the laser chip, or monitors light split by a beam splitter arranged in front of the laser chip, although not shown. FIG. 43 shows a typical single-loop feedback control system. Since a difference between the setup voltage V.sub.s 901 and the monitor voltage V.sub.m corresponds to the driving voltage V.sub.d 903, the light output is always controlled to be proportional to the setup voltage V.sub.s so as not to be varied due to a change in temperature.
In the prior art, however, since the semiconductor laser is driven and oscillated using an input current having a rectangular waveform, it is very difficult to design a circuit. In order to increase an extinction ratio (dynamic range) of a light output, assuming that a system for performing strength modulation by changing a pulse width/numbers of one picture element with a constant light output (pulse width/numbers modulation), or a system as a combination of the pulse width/numbers modulation and a change in light output (amplitude modulation) is adopted, a recording speed (picture element clock frequency) per picture element of a laser beam printer is as fast as several MHz. For example, if pulse-width modulation having 8-bit (256) gradation is performed, a minimum pulse width becomes very small, i.e., several nsec. When strength control of the semiconductor laser for generating such a very small pulse width is to be performed by the APC circuit with high precision, a control speed must be much increased to several tens of GHz. It is very difficult to perform such high-speed control, and a very expensive circuit is required to realize the high-speed control.
When a normal APC circuit having a stable control speed is used, the driving speed of the semiconductor laser driving circuit as a whole must be decreased, and high-speed pulse width/numbers modulation cannot be performed.
For these reasons, when a semiconductor laser is modulated to draw a halftone image, it is difficult to obtain a good multi-gradation image, e.g., a good halftone image having 256 gradation levels or more, and if possible, an increase in cost occurs.